The "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Fact on File, New York, 1997) states: "nitty-gritty - Getting down to the nitty-gritty is getting down to basic elements. Though first recorded in the 1960s the expression is probably older; the nitty-gritty of the phrase may be gritlike nits (small lice) that are difficult to remove from the hair or scalp."

Actually, a "nit" is a lice egg. But anyway.

The "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins, New York, 1977, 1988) sidesteps the whole lice issue and says nitty-gritty is a case of alliteration. "nitty-gritty means the basic elements of a matter, especially of a serious problem or challenge; the harsh truth. 'Get down to the nitty-gritty!' It seems to be a borrowing from black slang and is probably a reduplication of 'grit' and 'gritty.'"

"Listening to America" by Stuart Berg Flexner (Simon and Schuster, New York, 1982) gets down to the REAL nitty-gritty and says: ".Get down to the nitty-gritty, to get down to the hard facts or hard bargaining, 1963, when it was first popularized by black militants in the Civil Rights movement. (It may have referred to the gritlike nits or small lice that are hard to get out of one's hair and scalp or to a black English term for the anus.); it means the same as the English 'to get down to brass tacks'."